Secret signaling



June 14, 1927.

J. c. SCHELLENG SECRET SIGNALING- Filed Aug. 12. 1925 T o-aomw b MAW);

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Patented Jane 14, 1927.

UNITED STATES PATENT OFFICE.

JOHN C. SGEELLENG, OF MILLBUB NEW JERSEY, ASSIGNOR TO WESTERN ELECTRICcommnv, nzconrona'rnn, on NEW YORK, n. Y., A coaronarron or raw roan.sncanr srennme.

-App1ication filed August 12, 1925. Serial No. 49,785.

This invention relates to signaling systems and particularly toarrangements for providing secrecy in the transmission of messages oversuch systems.

An object of the invention is to-transmit signals which shall beintelligible only at a properly equipped receiving station.

A further object is to secure secrecy in message transmission withouthaving to add any other frequency components or utilizing a widerfrequency range than that comprised in the original message. a

The effectiveness of several promising systems of secret messagetransmission which have been proposed heretofore has been limited by thefact that it is exceedingly difficult in practice to provide a scheme ofmutilation which will enable the speech to be restored and understood atthe receiving station but to be practically unintelligible forunauthorized listeners. This difficulty in many of such systems seems tobe due in part to the fact that the voice frequencies within certainsmall ranges are transmitted in their original relationships withrespect to each other, and a certam amount of intelligibility isassociated with even a small range of frequencies. Therefore, in suchcases it is possible for an unauthorized listener, by using heterodynereception, to understand a few wordsof the transmitted messages.Moreover, imperfecw tions in the transmitting apparatus or in itsadjustment may permit a small amount of the original speech to betransmitted without change. I

In accordance with the present invention a message band such as speechis transmitted with secrecy within substantially the same 40' fre uencylimits as the original message ban This is accomplished by dividing themessage waves arbitrarily into sets of fragments on a time basis,reducing the frequency of each component of the successive ra ents by afactor equal to the number of ragments in a set raising the frequency ofeach component of at least one fragment in each set, superposing thevarious fragments of a set, the components of which are so changed infrequency, and transmitted to ether the superposed fragments within sustantially the frequency limits of the original message band.

' In'the particular embodiment ofthe in,-

vention to be described, a message wave is divided into successive timeintervals which are sent in rotation over different channels to a mamtransmission path. A s cial automatic devlce, such as a telegrap h one,inserted 1n each of these channels, first records the speech intervalsand then re roduces them at substantiall half s e so as to destroy theirintelligibility. he frequencles 1n one channel are then raised bymodulatlon so that the absolute frequency range of the frequencies inthe two channels i do not overlap. The two channels converge and the twobands of frequencies are transm tted together over a transmission pathby wire or radio, the total fre uency range of the transmitted fragmentsing the same as the range of the original speech.

At a receiving station means are provided for separating the twofrequency bands and for dlverting them into separate receiving channels.The raised frequencies in the freguency band in one channel are thenreneed to frequency by demodulation by the amount they were raised atthe transmitting station. Devices similar to those at the transmittingstation are utilized in each channel at the receiving station first torecord and then to reproduce the messa e fragments, these devicesbeing'arranged in this case, however, so that the fragments arereproduced at twice the speed of recording. The apparatus at thereceiving station is synchronized with that of the transmitting stationby suitable means so that the reproduced fragments in the two channelsat the receiving station, which fragments now have their originalfrequency values, may be supplied to a suitable receiving circuit intheir proper successive order so as to be intelligible.

The particular advantages and features of the invention are brought outmore clearly in the following detailed description when read inconnection with the accompanying drawing, in which Fig. 1 is a dia- 100grammat-ic illustration of one embodiment of the invention applied to acommunication system and Fig. 2 is an illustration in section of oneform of contacting device which may be used in the system of Fig. 1 10sto divide the message wave into fragments .on a time basis.

sion line ML is shown. The circuit at the transmission terminalcomprises a transmitter circuit T, adapted to be connected by means ofthe switch S with line ML through either one of two channels 1 or 2. Theoperation of switch S is controlled by clectromagnet 3 which isenergized by battcry 4. For the purpose of connecting transmittercircuit T in rotation to channels 1 and 2 for desired intervals of time,the battery circuit for electromagnet 3 is alternately made and brokenby means of the cont-actor device 5, which is driven at a desired speedby the synchronous motor SM in the manner which will be described later.

The contactor device 5 may be in theform of a commutator comprising, asshown in detail in the sectional view in Fig. 2, a metal portion 26 andan insulation portion 27, so arranged that the brushes 28 make contactwith the metal portion 26 for half a revolution of the commutator andwith the insulation portion 27 for the remaining half revolution of thecommutator.

An automatic device TGr which maybe of any form which is adapted forrecording speech intervals and reproducing them at substantially halfthe speed of recording is inserted in channel 1. The device TGrillustrated comprises a recording electromagnct 7, a thin steeltelegraphone disc 8, and a reproducing electromagnet 9. A similar deviceTG comprising the recording electromagnet 10, thetelegraphone disc 11and the reproducing electromagnet 12, is inserted in channel 2. Thediscs 8 and 11 are mounted on a shaft 13 which is rotated at a desiredspeed by the motor SM.

The recording electromagnets 7 and 10 are stationary and are mounted inclose proximity to the outer edge of discs 8 and 11, respectively. Asleeve spindle 14 having a sliding fit on shaft 13 is adapted, by meansof a train of gears 15, 16,17 and 18 of properly selected diameters, tobe driven by the motor SM in the same direction as disc 8 but atsubstantially half its speed. The reproducing electromagnet 9 of deviceTGr is mounted near the outer edge of disc 8 on crank arm 19, which isatlixed to gear 15. The gear 15 is mounted on the sleeve spindle 14,and, therefore the electromagnet 9 will rotate in the same direction asdisc 8 but at half its speed. The commutator of contractor device 5controlling the operation of switch S, is mounted on the sleeve spindle14 so that it rotates also at a speed which is substantially half thatof the discs 8 and 11. The reproducing magnet 12 of the device TGr inchannel 2 is mounted in close proximity to the outer edge of disc 11 ona crank arm 20 which is affixed to the gear 21. The gear 21 is freelymounted on shaft 13 and by means of the train of gears comprising gears'prime mark after the characters.

22, 17 and 18, is arranged to be driven by motor SM in thesame directionas disc 11 and at half its speed. The reproducing electromagnet 12 will,therefore, be rotated in the same direction as the disc 11 but at halfits speed.

The switch S. the operation of which is controlled by the sameelectromagnet 3 which controls the operation of switch S 'is adapted toconnect erasing battery 23 alternately to electromagnets 7 and 10. Thecontacts of switches S and S are so arranged that the erasinsr battery23 will always be connected to the particular channel to whichtransmitter circuit T is not con: nected.

The output leads of the revolving electro magnet 9 in channel 1 arearranged to be connected to line ML at all times by suitable means suchas the commutator device 24,

comprising a commutator ring 29 rotating with but insulated fromsleeve-spindle 14 and stationary brushes 30 making contact withcommutator ring 29. The output of reproducing electromagnet 12 inchannel 2, however is connected, through a commutator device 25 similarto device 24 in channel 1, to the input circuit of a modulator M. Asource of oscillations O is also connected to the input circuit ofmodulator M. The output circuit of modulator M is associated with lineML through a hand filter F The arrangement of the circuit at thereceiving terminal of the system is similar to that described for thetransmitting terminal and most of the apparatus used therein isidentical with that used at the transmitting terminal. The elements atthe receiving terminal corresponding to those at the trans mittingterminal have been des gnated with the same reference characters as usedat the transmitting station with the addition of a The circuit at thereceivin terminal differs from that at the transmitting terminal in thefollowing particulars:

A receiving circuit R, instead of a transmitting circuit T, is adaptedto be connected by the switch S with the channels 1' and 2'corresponding to the channels 1 and 2 at the transmitting station. Thestationary electromagnets 7' and 10 corresponding to the recordingelectromagnets 7 and 10 at the transmitting terminal, act at thereceiving terminal as reproducing magnets, while the movableelectromagnets 9 and 12', corresponding to the reproducingelectromagnets 9 and 12 at the transmitting terminal, act at thereceiving terminal as recording electromagnets. Between the line 'ML andthe contactor device 24' in channel 1' of the receiving terminal a bandfilter F is inserted. this filter being designed to pass the band offrequencies put on the line ML fromthe channel 1 at the transmittingterminal, and

! source of oscillations 0,, similar to the.

4 acsacee to exclude other frequencies. Similarly, a band filter F inchannel 2' is associated with line ML, this band filter having thesource of oscillations at the transmitting station.' The other elementsterminal are identical with the s milarly designated elements at thetransmitting ter-' mmal.

The band filters F at the transmitting terminal, and F, and F, atthereceiv ng terminal are used in the system for selectivelytransmitting waves of a particular range of frequencies and forsuppressing transmission of waves of other frequencies. These filtersmay be designed in accordance with the principles disclosed in CampbellPatent No. 1,227.113, dated May 22, 1917., The modulator M may be of thebalanced vacuum tube type, designed to suppress the unmodulated carriersuch as is illustrated and described in U. S. patent to Carson No.1,343,307, dated June 15, 1920. The demodulator D may be of the balancedvacuum tube type such as illustrated in- Fig. 49 of an' article entitledCarrier current telephony and tclcgra by published in the Transactionsof the 1 merican Institute of Electrical Engineers, vol. 40, 1921. Thesource of oscillations O and may be of any suitable type as for examplesuch as is disclosed in U. S. patent to Hartley No.

1,356,763, October 26, 1920.

Any of the well known methods may be used for synchronizing theapparatus at the transmitting and receiving terminals in the systemshown in Figure 1. In the particular one illustrated, a source of energyE 511 plying energy to operate a motor S M at t e transmitting terminalat a desired speed is also utilized to supply energv which istransmitted over an auxiliary line L to operate the motor SM at thereceiving terminal' in synchronism with the'motor S at the transmittingterminal.

The operation of the system of Figure -1 will now be described. Duringthe interval of time corresponding to a half revolution of thecommutator contactor 5, or one revo lution of the discs 8 and 11, thebrushes 28 make contact with the metal ortion 26 of contactor 5, thecircuit connecting battery 4 to electromagnet 3' thereby being closedand electromagnet 3 energized throughout this interval; The ener izationof electromagnet 3 causes switches 1 and S to be operated, and thecircuits are then arranged as shown in Fig." 1. With the circuits s0arranged the current through recording electromagnet 7 East thereproducing electromagnet 9 inat the receivingof the device TG, inchannel 1 will be varied in accordance with the message variationsreceived from transmitter circuit T. The recording electromagnet 7impresses these cur-' rent variationson the telegraphone disc 8, and apermanent magnetic record of the variations are made on the disc 8 as itrevolves past the stationary electromagnet 7 The magnetic record on thedisc 8 as it revolves uces a current in that electromagnet, whichas'this electromagnet is rotated in the same direction as the disc butat half its speed, will have a frequency range one-half that of theoriginal variations received from transmitter circuit T, that is, eachfrequency v inthe frequency spectrum of the original} message variationshas been reduced by this process to one-half of its original value.After one com lete revolution of the disc 8, or half a 'revo ution ofthe commutator of contactor 5, the magnetic record of the messagevariations received from transmitter circuit T extends completely aroundthe disc 8, and only one-half of this record has been reproduced as thereproducing electr0mag-. net 9 is only revolving at one-half the speedof disc 8. At the end of the interval, the brushes 28 of contactor beginto make contact with the insulated portion 27 of contactor 5, theconnection of battery 4 to electromagnet 3 being thereby broken so thatelectroma net 3 is de-energized. The switches 1 and S are thereby thrownin the .opposlte direction from their previous position so thattransmitter circuit T is .disconnected from the recording electromagnet'7 of the device TGr in channel 1 and connected to the recordinelectromagnet 10 of device TGr in channe 2, and erasing battery 23 isdisconnected from the recording electromagnet 10 of device TGr andconnected to the recordin electromagnet 7 of de-- vice TGr in channed 1.During the succeeding interval while the contactor 5 is making the lasthalf of its revolution, disc 8 is making its second revolution andtheunrepr'oduced ortion of the magnetic record on that disc is eingreproduced at half speed by the reproducing electromagnet 9. Meanwhile,the erasing battery 23 is acting on electromagnet 7 so as to eiface thereproduced portion of the magnetic record on disc 8. At the end of thesecond revolution of disc 8 the magnetic recordmade thereon during 120the first revolution has been completely reproduced and erased from thedisc. Meanwhile, during' the interval corresponding to the last halfrevolution of the commutator of contactor 5.and the second revolution of125 disc -8, themessage variations from the transmitter circuit T arebeing impressed on the recording electromagnet 10 of the device T622 inchannel 2, recorded on the disc 11 and reproduced at half speed by the.reproducing electromagnet 12 in the same manner as described inconnection with channel 1. When the magnetic record on disc 11 extendscompletely around disk 11, the switches S and 553 are thrown again inthe opposite direction due to the brushes 28 again making contact withthe metal portion 26 of contactor 5, and the cycle described above isrepeated.

It will be apparent from the above that while the inputs to thefi-ecordinglectromagnets 7 and 10 in channels 1 and 2 respectivel due tothe switching operation describe consisting of alternate blank andmessage intervals, the message variations in the outputs of reproducingmagnets 9 and 12 in the respective channels will be continuous having afrequency range of one-half that of the ori inal message variations.

As the outputs o the two reproducing electromagnets 9 and 12 coverthefsame absolute frequency range, to avoid overlapping when they aretransmitted over the line ML, the

variations in the output of magnet 9 in channel 1 is impressed directlyon the line ML as shown, while the frequencies of the variations in theoutput of magnet 12 in channel 2 are raised by modulation before beingimpressed on the line ML.

It will be assumed for purposes of description that, as indicated in thefigure, the transmitter circuit T generates message currents rangingfrom 0 to 3000 cycles, that oscillator 0 in channel 2 supplies a currentof 1500 cycles to the input circuit of modulator M, that band filter Fat the transmitting terminalhas a transmission range of 1500-3000cycles, that band filters F and F in channels 1' and 2' respectlvely, atthe receiving terminal have transmission ranges of 0-1500 cycles and1500-3000 cycles, respectively. and that oscillator 0 at the receivingstation supplies a current of 1500 cycles to the input circuit ofdemodulator D. Y

The half-speed message variations in the output of re roducing magnet 9in channel 1 will, there ore, have a frequency range of 0-1500 cyclesand, as shown in the Flgure 1, these variations are transmitted directlyto the line ML. The half-speed message variations in the output ofreproducing magnet 12 in channel 2 will also have a frequency range of0-1500 cycles. These vanations are then combined in modulator M with thecurrent of 1500 cycles from the oscillator 0 and a resultant band of1500- 3000 cycles produced which is vselectively transmitted by filter Fand sent out over the line ML. The total frequency range of the speechvariations transmitted over theline ML including the frequency band of04500 cycles from channel 1 and the fre- 1500-3000 cycles fromchanquency band of the same as the frequency nel 2, is, then,

the recording electromagnet 9' of device TGr in that channel. The bandof 1500- 3000 cycles selectively transmitted by filter F in" channel 2iscombined in the demodulator Dwith the current of 1500 cycles from theoscillator 0 thereby reducing the frequenc1es in the band by the sameamount that they were raised by modulation in the modulator M at thetransmitting terminal. The resultant frequenc band 0150-1500 cycles inthe output 0 demodulator D is ilrp pressed on the recording,electromagnet of device TG' in channel 2.

The

message variations supplied to recording electromagnets 9 and 12 inchannels 1 and 2, respectively,'are recorded'on discs 8 and 11respectively, and reproduced by the reproducing electromagnets 7' and 10scribed in connection with the operation of the similar elements at thetransmittingterminal. In this case, the reproducing magnets 7 and 10'are stationary and the recording magnets 9 and 12 are rotated by motorSM, in the manner described for the corresponding elements at thetransmit- .tlng terminal, in the same direction but at half the speed ofthe telegraphone discs 8 and 11. By the recording andreproducelectromagnets 7 and 10 at the transmitting terminal.

As the motors SM and SM at the trans mitting and receiving terminalsrespec tively, are operated in synchronism in the manner described ifthe phase relations of the elements of contractor device 5' and '9respectively, in the same manner as was detelegra hone devices TG' andTG, are

proper y adjusted withrespect to the similarly designated elements atthe transmitting terminal, the message intervals in the output of thereproducing electromagnets 7 and 10 at the receiving terminal by theoperation of switch S aresupplied to the receiving clrcuit R in theproper order .to reproduce the same intelligible speech varia- ,ofcourse,

tions as generated by the transmitter circuit T at the transmittingterminal.

Although the invention has been described and illustrated in connectionwith its use in wired transmission systems, it 1s,

applicable as well'to radio transmission. It is to be understood thatthe scopeof 'the invention is defined in the following claims, and thatthe invention 1s not band of frequency components, whlch method consistsin dividing sa1d-message waves arbitrarily into sets of fragments on atime basis, reducing the frequency of each component of the successivefragments by a factor equal to the number of fragments in a, set,superposing the various fragments of a set, the components'of whicharechanged in frequency, and transmitting together the super osedfragments.

2. he method ofsecuring secrecy in I transmission of a band .of messagewaves within substantially the frequency limits of said band comprisingdividing said message waves arbitrarlly into sets of fragments on a timebasis, reducing the frequency of each component of the successivefragments by a factor equal to the number of fragments in a set,superposingthe various fragments of 'a set the components of which arechanged in frequencyfrand transmitting together the superposed agmentswithin substantially the dfrequency limits of the original message an 3.The method of secret transmission of message waves having a band offrequency components, which comprises dividing said time basis, means toreduce the re message waves arbitrarily into sets ,of fragments on atime basis, reducing the fre uency of each component of the successiveragmentsv by a factor e ual to the number of fragments in a set, c aning the frequency of each component in at east one fragment of each setso as to produce therein a band of frequency components lying outsidethe equency range 0 the bands of components of reduced frequency in theother fragments of the'set, superposing the various fragments of theset, and transmitting the superposed fragments within substantially thefreuency limits of the original message band.

4 .1 In a secret signaling system, a source of message waves comprisinga band of frequency components, means for dividing said message wavesinto sets of fra ments on a uency of each component of the successiveagments to substantially one-half of 'its original value, means toincrease the frequency of of. which have each component in at least onefragment of of message waves comprising a band of. frequency components,means for dividing said waves arbitrarily into sets of fragments onatime basis, means to reduce the frequency of each-component of thesuccessive frag- 5. Ina secret signaling system, a source ments byafactor equal to the number of' fragments in a set, means to superposethevarious fragments of a set, of wh ch are cha'n posed fragments.

6 Ina secrecy signaling system, means to generate message wavescomprising a band of frequency component means for dividing said messagewaves arbitrarily into sets of fragments on a time basis, means toreduce the frequency of each component of the successive fragments by afactor equal to the number of fragments in a set, means to superpose thevarious fragments of a set, the components of which are changed infreuency, and means for transmitting together 1; e superposed fragmentswithin substantially the frequency limits of the original message band.

7 A secret signaling system comprising a transmitting station, areceiving station and a transmission path therebetween, saidtransmitting station comprising means to generate message currentshaving a band of frequency components, means to arbitrarily divide saidmessage currents into sets of fragments on a time basis, means forreducing each frequency component in the fragthe components 4 ed infrequency, and: means fortransmittingi together the superon sa1dreceiving device in proper order to reproduce the original message.

v 8. In a secret signaling system, a transmita receiving station, and atrans therebetween, said transmitting ting station, mission path stationcomprising means for generating a message wave having a band offrequencies, means for separating said message waves into successivefragments on a time basis,

means for recording said successive fragments, and means finreproducingthe recorded fragments, and transmitting them to said transmission pathat a speed substantially half the speed at which said fragments weregenerated, said receiving station comprising a receiving device, meansfor recording the message fragments received over said.

line, means for reproducing the recorded fragments at a speedsubstantially "equal to the speedat which they were recorded at thetransmitting station, and means to impress the reproduced fragments onsaid receiving device in the successive order in which they were roducedat the transmitting station.

9. if secret signaling system comprising a transmitting station, areceiving station, a transmission path therebetween, a pair of signalingchannels at said transmittmg station, means to generate a message wavehaving a band of frequencies, means to direct successively producedportions of said wave alternately into said signalingu channels, meansin each of said channels toreduce the frequency components in thereceived message portions to substantially one-half of their originalvalues to make said message nels on said transmission path, a pair ofre-' ceiving channels at' said receiving station,

means in said receiving channels for selecting said different frequencybands received over said transmission path, means in one the selectedfrequenc band therein so as to produce equivalent requency bands in thetwo receivmg channels, means to' increase each frequency in theequivalent frequency bands in the twovchannels to substantially twiceits value, a receiving circuit and means to impress the bands ofincreased frequency in said receiving channels on said receiving c1rcu1tso as to reproduce the original message wave.

10. A secret signaling system comprising a transmitting .station,--"a;receiving station, a

transmission path therebetween, a air of signaling channels at saidtransmitting station, a source of signaling variations, means foralternately connecting said source to said channels, means in each ofsaid channels for recording the received signaling variations, andreproducing them at substantially half the speed of recording, means formodulating a carrier wave in accordance with the reproduced variationsin one of said chan;

nels to produce a band of frequencies lying outside the range of theband of reprodu frequencies in the other of said channels means forimpressing the different bands 0 channels, and reproducing them attwicethe speed of recording, a receiving device and means foralternately connecting the outputs of said receiving channels to saidreceiving device so as to reproduce the original signaling variations.

In witness whereof, I hereunto subscribemy name this 6th day of Au ofsaid receiving channels for demodulating 4 JOHN C. .S

st A. n. 1925. HELLEiIG.

